That's what we are specifically pinpointing in this thread, though--a relationship between 2 adults, one that happens to be a teacher and one that happens to be a student.
And my point is that specific, narrow question doesn't have studies (that I can see) targeting it precisely.
I'd say you are on firmer ground comparing it to the abuse of power as sometimes seen in Employer-Employee relationships or Officer-Soldier relationships.
I have made a wide variety of comparisons, including those, precisely so that a wide variety of plausible comparisons can be made.
A better default position would be to not assume that there is any inherrent negative or positive outcomes between consenting adults in these scenarios (i.e., individuals 18 or older in committed sexual relationships where there is an existing power discrepency), even though in exceptional cases there may be.
Why would that be the default position? We have, as already shown, significant evidence that outcomes are negative in aggregate, to a statistically significant degree.